So, we got to building his motivation to murder the leader of the bullies. I suppose the beginning of the manga was sort of a balance between a slow start and the need to get things rolling. If it takes too long for him to murder the first victim, the whole series would be kind of pointless, but if he just summarily kills people left and right, it would have little to no impact. It was actually said he wants to kill Yamada the most, but since she's the FMC, I presume, I imagine she might not die at all in the story.
@Velsy She will obviously think he drew a picture of her in a fancy dress. What she might think of such fanart (she's a part-time model, after all, so chances are she thinks it's fanart), is another question. At least she didn't show it to the other girls and start to laugh at it.